There are 88 constellations: Who’s who and what’s where?

Greetings stargazers. When people tell me they want to “learn” the night sky, what they often mean is they want to learn to identify the constellations they can see.

There are 88 constellations in the sky, including both the Northern and Southern hemispheres, so at any one time you can see approximately half, or 44, constellations in the celestial hemisphere above you. Since the Earth is rotating, during the night while constellations are setting in the west, more are continually rising in the east. That means that on any given night it is possible to see significantly more than 44 constellations.

There isn’t any part of the sky that is not within a constellation. The boundaries between constellations are somewhat arbitrary, imaginary lines that are sort of like the boundaries for counties within a state. The boundary had to be put somewhere. You don’t have to be in a city or town, but if you live in Colorado you are inside one county or another.

While many of the constellations we have defined today can trace their origins to prehistoric times, some are much more recently defined, simply in order to “fill in the gaps” in the sky.

Not all constellations are bright, or easily recognizable, even if you know where to look. Even some familiar constellations contain dimmer or more obscure parts.

The term that means a recognizable pattern of stars is not “constellation,” but “asterism.” An asterism could (and often does) correspond to the brighter stars in a single constellation. However, an asterism could be only part of a constellation, or it could contain stars in multiple constellations.

I can best explain the difference between constellations and asterisms with examples. The recognizable asterism called the Big Dipper is only a part of the much larger region in the sky contained within the boundaries of the constellation Ursa Major. The dipper is just the body and extra long tail of the “greater bear.” The legs and head of the bear extend well beyond the dipper.

Another asterism is the summer triangle, comprising the bright stars Vega, Deneb and Altair. Those three stars are in the constellations Lyra, Cygnus and Aquila, respectively.

Any star chart can help you learn some asterisms, but some may be easier to use than others. One disadvantage of “paper” star charts is that they usually contain “stick figures,” connecting the brighter stars. And not all charts agree on how these stars should connect. Although I can see the patterns, I have yet to really see any of the “sticks” in the sky. If you have an app that can toggle the stick figures on and off, that is one way to help you see the patterns and to learn how someone else thought the stars were connected.

This month

Besides the asterisms mentioned above, one asterism to look for this month is the scorpion that corresponds to the constellation Scorpius. Looking to the south, about 2˝ fist widths above the horizon, you can find the bright red star Antares as the heart of Scorpius. This constellation is one of the few that we, living in the 21st century, can recognize the same way as the ancient civilizations that named this constellation did. At least I think it looks like a scorpion. If your view to the south is not blocked by mountains or trees, you can see the tail of the scorpion swooping down and to the left toward the horizon, then hooking back up with the two stars making up the “stinger” only a little more than a fist width above the horizon.